1. Field of the Invention
This invention relates to an enclosed control device comprising a casing enclosing an electronic circuit in a closed state and cooling means for cooling the electronic circuit.
2. Description of the Prior Art
Variable speed power induction motors have conventionally been known as inverter motors and have capacities ranging between 0.4 and 3.7 kW. The inverter motor comprises an electric motor and an inverter assembly or an inverter mounted integrally on the motor. The inverter comprises a metal casing mounted to a motor frame and an inverter circuit or electronic circuit disposed in the casing.
The power motors are sometimes used in severe environments. For example, the power motors are used in the water as submersible pump motors, mounted as a power source on a machine producing a large amount of dust, or trampled down by muddy feet of operators. Accordingly, the inverter casing mounted on the motor frame requires the same strength as the motor frame, dust proof, and waterproof in the inverter motors serving as the above-described power motors. The electronic circuit heats up in the casing of the inverter assembly of the above-described type or the enclosed control device. The heated electronic circuit needs to be cooled. In the prior art, the electronic circuit have been cooled by natural radiation from the casing or a cooling device such as a fan is provided outside the casing for force-cooling the casing so that air in the casing and accordingly the electronic circuit are cooled.
In the above-described case, however, the casing requires the same strength as the motor frame. This increases the wall thickness of the casing and also raises its heat resistance. In the prior art, both the natural radiation and the forced radiation depend upon that heat in the casing is transmitted by natural convection to an upper wall of the casing. Consequently, the electronic circuit is not sufficiently cooled. This reduces the life of the product.